for loading the file content you can use the code specified below
FileStream fStream = new FileStream("c:\\myfile.txt", FileMode.Open);
richTextBox1.Selection.Load(fStream, DataFormats.Text);
for saving the content you can use the code specified below
TextRange range;
FileStream fStream;
range = new TextRange(richTB.Document.ContentStart, richTB.Document.ContentEnd);
fStream = new FileStream("c:\\myfilenew.txt", FileMode.Create);
range.Save(fStream, DataFormats.Text);
fStream.Close();